The spatiotemporal distribution of Foxi1-expressing pulmonary ionocytes in the mouse airways

Pulmonary ionocytes are rare epithelial cells that bear the highest amount of mRNA for CFTR and whose function is necessary to regulate airway surface liquid homeostasis. Lineage induction is regulated by the transcription factor forkhead box i1 (Foxi1), but the temporal and spatial localization of Foxi1⁺ cells during airway development and the impact of CFTR mutations remain poorly understood. Here, we used immunofluorescence and RNAscope to detect Foxi1⁺ cells throughout postnatal airway development in male and female mice, from neonatal stages to 1 yr of age, including Cft^rΔF508 mutant animals at the adult stage. Foxi1⁺ cells were observed in higher density in the laryngeal subglottis region, decreasing toward the distal trachea, and principally located in the surface epithelium covering intercartilage zones. The Foxi1⁺ cells were observed at postnatal day 7.5, and their appearance correlated with the development of submucosal glands. The distribution of these cells was not altered in the Cft^{rΔF508/ΔF508} mutant mouse. These findings demonstrate that pulmonary ionocytes display a highly regionalized distribution pattern in the mouse airway and emerge during a defined postnatal developmental window associated with submucosal gland maturation. Their localization near submucosal gland openings in the mouse trachea and coappearance during development suggest a potential role in regulating submucosal gland secretion and airway epithelial fluid homeostasis.NEW & NOTEWORTHY This study provides the first spatiotemporal map of Foxi1⁺ pulmonary ionocytes in mouse airways. These rare cells, which express the majority of CFTR mRNA, emerge postnatally near developing submucosal glands and concentrate in the laryngeal subglottis during adulthood. Their distribution remains unaltered in Cftr^{ΔF508/ΔF508} mice, suggesting CFTR dysfunction does not disrupt ionocyte patterning.